Epics often used archaizing language to give the impression of antiquity and elevate the tone. Using slang alongside that would be contradictory, so I would expect it to not happen much. As for swear words, I wouldn't be surprised to see some in the old Norse sagas or from around the Mediterranean, but in the Anglo-Saxon literature we have, and in Beowulf in particular, the author was trying hard to be spiritually uplifting, so I'd expect to not see any.